The factors involved in song development and the function of avian dialects have been the subject of a great deal of recent research and speculation. This research has explored several issues that are central to ethological theory, such as nature of sensitive phases, sensory templates, and sex differences. There have also been attempts to develop the implications for linguistids, speech therapy, and language acquisition in humans, and many parallels have been drawn between dialect systems in birds and humans. Four specific questions are addressed in this proposal. 1. The length of sensitive phases. 2. The range of song inputs that can be learned. 3. Modifiability of learned song. 4. Sex differences in any of the above characteristics.